Dicarba-closo-dodecaborane (hereinafter abbreviated as “carborane” in the specification) is an icosahedral cluster containing two carbon atoms and ten boron atoms in which both atoms are hexacordinated. In caboranes, depending on the position of the carbon atoms in the cluster, 3 kinds of isomers exist, i.e., 1,2-dicarba-closo-dodecarobane (ortho-carborane), 1,7-dicarba-closo-dodecaborane (meta-carborane), and 1,12-dicarba-closo-dodecaborane (para-carborane). These structures are unique among boron compounds, namely they are characterized to have very high thermal stability and hydrophobicity comparable to hydrocarbons.
A major utility of compounds composed of a carborane so far has been an application to 10Boron-Neutron Capture Therapy (BNCT). 10Boron-Neutron Capture Therapy has been developed as a therapy mainly to glioma and melanoma. When 10B atom is irradiated with thermal neutron (slow neutron), an a ray with 2.4 MeV energy is emitted and the atom is decomposed to 7Li and 4He. The range of α ray is about 10 μm which corresponds to a diameter of cells. Therefore, effects are expected that only cells in which 10B atoms are uptaken are destroyed and other cells are not damaged. For the development of BNCT, it is important how to have cancer cells selectively uptake 10B atoms in a concentration capable of destroying cells with neutron radiation. For that purpose, ortho-carborane skeleton has been utilized which has low toxicity and a high 10B atom content, and is easy to be synthesized. Moreover, nucleic acid precursors, amino acids, and porphyrins which contain ortho-carboranes have been synthesized and subjected to evaluation.
Recently, physiologically active compounds having a carborane skeleton as a pharmacophore have been reported (International Patent Publication WO00/43016). These compounds have physiological activities of retinoid actions, anti-estrogen action and the like, and they are useful for the treatment of leukemia and the like.